An electronic component is mounted on a printed circuit board or the like. In general, the electronic component is mounted through a so-called reflow method. Particularly, the electronic component and the printed circuit board or the like are at first temporarily bonded to each other with solder balls for semiconductor packaging (referred to as “solder balls” hereunder) and a flux provided therebetween. Next, the entire printed circuit board is heated so as to melt the solder balls, followed by cooling the corresponding printed circuit board to a normal temperature so as to solidify the solder balls, thus ensuring solid solder-bonded sections.
It has been increasingly required that a solder alloy used as a connection material in an electronic device be made lead-free so that a negative impact on the environment may be minimized when disposing the corresponding electronic device. Accordingly, as a composition of a solder bail, there are extensively used, for example, an Sn—Ag eutectic composition (Ag: 3.5% by mass, Sn: remainder), and a solder composition disclosed in Patent documents 1 and 2 in which a small amount of Cu serving as a third element is added to the aforementioned Sn—Ag eutectic composition. Further, a solder ball having a composition similar to that described above is mainly used also in a BGA (Ball Grid Array) which has been practiced significantly widely in recent days.
When operating an electronic device, a heat is generated inside the electronic device due to a current applied for operation. The aforementioned solder ball is used to connect materials with different thermal expansion coefficients, such as a silicon chip, a resin substrate or the like. Accordingly, the solder ball is subjected to a thermal fatigue environment upon the operation of the electronic device. As a result, cracks may progress inside the solder ball, thus posing a problem of interfering with a receiving/sending of an electrical signal through the solder ball. In addition, mobile electronic devices have been rapidly reduced in size and weight in recent years. That is, with regard to a printed board or an integrated circuit substrate used in an electronic device in such case, a bonded area of each solder-bonded section used in an electronic member is shrunk, thus making it more important than ever to improve a thermal fatigue property. With regard to a solder ball formed into a conventional size, i.e., when the solder ball has a diameter of not smaller than 300 μm, an area of a bonded section composed of the solder ball and an electrode is sufficiently large. Accordingly, in such case, a decrease in a bonding strength between the solder ball and the electrode does not become a problem, and the thermal fatigue property is not adversely affected, even if an oxide layer on a surface of the solder ball fails to be completely removed during a reflow process and partially remains in a bonded interface. Here, in order to ensure the thermal fatigue property of the conventional Sn—Ag—Cu-based solder ball having the diameter of not smaller than 300 μm, it has been considered as effective to set an Ag concentration to about 3 to 4% by mass instead of ensuring the bonding strength of the bonded interface by removing the oxide layer on the solder ball surface. This is due to the following speculation. That is, an increased Ag concentration can allow an intermetallic compound called Ag3Sn to be precipitated in a large amount in the solder ball, thereby hardening the solder ball through a precipitation hardening effect, and thus making it difficult for the solder ball to be deformed by an external force. In this way, a displacement resulting from the thermal fatigue can be small even when a load occurs as a result of the corresponding thermal fatigue, thus making it possible to slow a crack progress inside the solder ball.
Further, it is required that the solder ball ensure: a wettability at the time of soldering; a low melting point such that the soldering can be performed at a temperature that is as low as possible; a surface quality such that a device on which the solder ball is mounted can correctly recognize the corresponding solder ball through image recognition at the time of mounting the same; and a drop resistance such that an electronic device exhibits no failure even after being inadvertently dropped.